The present invention relates to hydrides of lithiated nickel dioxide having X-ray diffraction patterns essentially equivalent to lithiated nickel dioxide having a hydrogen-free crystal lattice. Secondary electrochemical cells incorporating hydrides of lithiated nickel dioxide as the cathode-active material are also disclosed, which demonstrate a significant increase in reversible capacity compared to cells incorporating as the cathode-active material lithiated nickel dioxide having a hydrogen-free crystal lattice. Methods of preparing the hydrides of lithiated nickel dioxide of the present invention are also disclosed.
Electrochemical cells useful as electrical storage batteries usually incorporate a metal-containing anode and a cathode including an active material which can take up ions of the metal. An electrolyte incorporating ions of the metal is disposed in contact with the anode and the cathode. During discharge of the cell, metal ions leave the anode, enter the electrolyte and are taken up in the active material of the cathode, resulting in the release of electrical energy. Provided that the reaction between the metal ions and the cathode-active material is reversible, the process can be reversed by applying electrical energy to the cell. If such a reversible cathode-active material is provided in a cell having the appropriate physical configuration and an appropriate electrolyte, the cell can be recharged and reused. Rechargeable cells are commonly referred to in the battery art as "secondary" cells.
It has long been known that useful secondary cells can be made using a light alkaline metal such as sodium, potassium and particularly, lithium, as the source of the metal ions exchanged between the anode and cathode through the electrolyte. These metals are particularly useful in combination with a cathode-active material that is a sulfide or oxide of a transition metal, i.e., a metal capable of assuming plural different valence states. In the past, these alkaline metals such as lithium have been used in electrochemical cells in their pure metal state as the cell anode in combination with the transition metal cathode-active material. See, for example, Dampier, J. Electrochem. Soc., 121(5), 656-60 (1974). It is common knowledge that water reacts with alkaline metals such as sodium, potassium and lithium in their pure metal state, reducing the suitability of these metals as electrode materials. Therefore, extreme care must be taken during cell assembly to avoid exposure of the anode metal material to ambient moisture and other sources of water.
Secondary lithium cell researchers have sought to develop a rechargeable lithium cell containing no metallic lithium. Cells have been developed using instead of a lithium metal anode, a lithium intercalation host that operates near the potential of lithium, such as the material in cells incorporating same disclosed in presently co-pending U.S. patent application Ser. No. 350,396 by Fong et al., filed May 11, 1989, which with the present application is commonly owned. The disclosure of which application is hereby incorporated herein by reference thereto.
Replacing lithium metal anodes with lithium intercalation host anodes removes some of the restrictions lithium metal anodes place upon cell design in choice of electrolytes and also the adverse effect lithium metal plating places upon cycling performance and safety in the finished cell. However, a source of lithium must still be supplied to the cell for exchange between the anode and cathode-active material through the electrolyte. This can be done by assembling cells with a sacrificial strip of lithium placed in electrical contact with the anode so that when electrolyte is added, the lithium is consumed by reacting with the intercalation host material of the anode. However, this wastes space and reduces cell capacity.
A preferred solution is to use a cathode-active material that already contains the required lithium. Lithiated nickel dioxide is considered to be a commercially feasible lithiated cathode-active material because it demonstrates sufficient reversible capacity over a voltage range of about 3 to about 4.2 volts. Lithiated nickel dioxide is also a useful cathode-active material in conventional lithium cells.
Japanese Published Patent Application No. 63-121,260 and European Patent Application Publication No. 243,926 disclose the preparation of lithiated nickel dioxide for use in lithium batteries by the solid state reaction of powdered nickel carbonates and/or oxides at temperatures in excess of 850.degree. C. in air. Japanese Published Patent Application No. 60-74,272 discloses a nickel hydroxide coating electrochemically oxidized in a lithium hydroxide solution to obtain a "lithium doped nickel oxide" that is then heat treated at 450.degree. C. for one hour.
U.S. Pat. No. 4,567,031 discloses a lithiated nickel dioxide for use as a cathode-active material having the formula Li.sub.x Ni.sub.y O.sub.z wherein x is between 0.1 and 1.1, y is between 1.1 and 0.1 and z is between 1.9 and 2.1, which is prepared by co-crystallizing or co-precipitating a stoichiometric solution of an oxygen-containing lithium salt and an oxygen-containing nickel salt. The resulting mixed salt is calcined at 400.degree.-500.degree. C. in a stream of air or a stream of carbon monoxide and carbon dioxide. The low temperature calcination is disclosed as producing a high surface area powder. Japanese Published Patent Application No. 63-19,761 discloses the preparation of lithiated nickel dioxide by the anodic oxidation of nickel hydroxide in an aqueous solution of lithium hydroxide. The lithiated nickel hydroxide is then washed in hot water and heated at 200.degree. C. for two hours to dry the material and drive off water to form the nickel dioxide. Lithiated nickel dioxide cathode-active material having the formula Li.sub.x Ni.sub.y O.sub.2, with x less than one and y about equal to one is also disclosed in U.S. Pat. No. 4,302,518.
Published European Patent Application Publication No. 345,707 discloses the preparation of lithiated nickel dioxide for use as a cathode-active material having the formula Li.sub.y Ni.sub.2-y O.sub.2 with 0.84.ltoreq.y .ltoreq.1.22, made from lithium hydroxide and nickel oxide, pulverized and mixed in stoichiometric ratio and heated in air to a temperature between 600.degree. C. and 800.degree. C. An excess of lithium hydroxide is used to compensate for volatilization of this material at the heating temperature. The material is disclosed as being useful as a cathode-active material for secondary cells.
Electrochemical cells having lithiated nickel dioxide as the cathode-active material typically have poor cycling capacities. In addition, lithiated nickel dioxide is thermally unstable when lithium is de-intercalated upon charging of the cell. The de-intercalation forms Li.sub.1-x NiO.sub.2. As x increases, the nickel approaches an unstable 4.sup.+ valence, and the material releases oxygen when heated. If a charged cell is welded on the positive electrode and local heating of the de-intercalated lithiated nickel dioxide occurs, oxygen can be liberated, which oxidizes the cell electrolyte solvent, generating more heat. If the "self-heating" rate becomes high enough, the cells can undergo thermal runaway, rupture and burn. This is not only a problem during welding of the cell casing. The self-heating reaction occurs at temperatures around 100.degree.-200.degree. C., which can result from typical electrical or thermal abuse of the cell.
Even when care is taken not to thermally release oxygen from the lithiated nickel dioxide charged cells, there is a tendency for gaseous products to accumulate with cycling, leading to a hazardous pressure buildup. These gaseous products are formed from impurities in the lithiated nickel dioxide such as lithium hydroxide (LiOH) and lithium carbonate (Li.sub.2 CO.sub.3) when the cells are charged under normal operating conditions. When sufficient levels of impurities are present, the pressure eventually accumulates to a level that actuates the pressure vent, a safety device that prevents a hazardous pressure buildup capable of bursting the cell. Nevertheless, activation of the pressure vent causes the cell to malfunction.
Parent U.S. patent application Ser. No. 556,754 discloses a lithiated nickel dioxide cathode-active material having improved cycling capacity, thermal stability and freedom from the evolution of gaseous products with cycling, compared to the existing art, having the formula Li.sub.x Ni.sub.2-x-y MyO.sub.2, with x being between about 0.8 and about 1.0, M being one or more metals selected from cobalt, iron, titanium, manganese, chromium and vanadium, and y being less than about 0.2, with the proviso that y is less than about 0.5 for cobalt. The lithiated nickel dioxide disclosed is obtained by heat treating a substantially homogeneous dry intermediate mixture of a starting material containing NiO, Ni(OH).sub.2 or mixtures thereof, and optionally including one or more oxides or hydroxides of a transition metal selected from cobalt, iron, titanium, manganese, chromium and vanadium, together with about a 10% to about 25% stoichiometric excess of LiOH at a temperature above about 600.degree. C. in an atmosphere substantially free of carbon dioxide and having a partial pressure ratio of oxygen to water vapor greater than about 15. Any LiOH or Li.sub.2 CO.sub.3 present is then removed from the heated mixture so that the lithiated nickel dioxide is substantially free of LiOH and Li.sub.2 CO.sub.3
The excess of LiOH ensures that x for Li will be between about 0.8 and about 1.0. The use of an atmosphere substantially free of carbon dioxide and having a partial pressure ratio of oxygen to water vapor greater than about 15 minimizes the formation of LiOH and Li.sub.2 CO.sub.3, which are not cathode-active, and which also scavenge lithium from the reaction mixture, thus depressing the value of x for Li. Furthermore, LiOH and Li.sub.2 CO.sub.3, when present in lithiated nickel dioxide, decompose electrochemically at high cell voltages. The LiOH generates oxygen, hydrogen and hydrogen peroxide, and the Li.sub.2 CO.sub.3 generates carbon dioxide and oxygen. These predominantly gaseous products lead to pressure buildup in the cells that eventually results in cell malfunction. By minimizing the formation of LiOH and Li.sub.2 CO.sub.3 and removing any LiOH and Li.sub.2 CO.sub.3 present, not only is the value of x for Li maximized, the accumulation of gaseous products causing pressure buildup in cells is significantly reduced to the point of elimination.
Parent U.S. patent application Ser. No. 556,754 discloses that any LiOH and Li.sub.2 CO.sub.3 that does form are preferably removed by a controlled water extraction, which must be done with care because hydrogen can replace the lithium in Li.sub.x Ni.sub.2-x-y MyO.sub.2 to make Li.sub.x-z H.sub.z Ni.sub.2-x-y M.sub.y O.sub.2. This application discloses that the displacement of lithium by hydrogen is not favored because the lithium is removed from the crystal lattice and replaced by hydrogen. The hydrogen is not exchanged between the cathode and the anode like the lithium. This application discloses that Li.sub.x-z H.sub.z Ni.sub.2-x-y M.sub.y O.sub.2 having z less than about 0.02 works well as a cathode-active material.
While the lithiated nickel dioxide cathode-active materials disclosed by parent U.S. patent application Ser. No. 556,754 have improved cycling capacity, thermal stability and freedom from the evolution of gaseous products with cycling, compared to the existing art, the cycling capacity remains somewhat marginal in the context of commercial feasibility. Therefore, a need exists for a lithiated nickel dioxide cathode-active material having improved cycling capacity, yet possessing thermal stability and freedom from the evolution of gaseous products with cycling possessed by the lithiated nickel dioxide cathode-active materials of parent U.S. patent application Ser. No. 556,754.